On 7th May Gwen found this metallic blue-black fly lying upside down on a window-sill in her house at Torbreck (NC0824; photo 1). It was about 10mm long and looked like one of the blowflies, members of the family Calliphoridae, the larvae of most of which feed on carrion. However, it did not fit illustrations of any of the commoner species, the bluebottles and greenbottles.
True flies (Diptera) are a challenging group to identify, with some 7100 species found so far in the British Isles, so we normally go to a specialist for help. Robert Wolton, who lives in Devon, but visits Assynt from time to time, kindly provided a name after we had posted the specimen to him.
It was a blackbottle or northern blowfly Protophormia terraenovae, the most cold-tolerant of the family (photo 2). It is indeed a carrion-feeder and may have originated from a sheep that died earlier this year on a neighbouring croft, just outside the garden fence.
Although NBN shows scattered records of the blackbottle across the North-west Highlands, there were no previous ones from Assynt.
If you are squeamish look away at this point. The larvae of this species are particular in that they feed selectively on necrotic tissue and will not transfer their attentions to adjacent healthy tissue. Sterile cultures are therefore used to cleanse and disinfect chronic wounds. For the followers of grittier crime dramas, the species is also used in forensic entomology to age corpses.
For those with a taste for further arcane information, Wikipedia relates that pupae identical to those of this species were found in the fossil skull of a steppe wisent (extinct bison), some 75,000 years old, that was excavated in 1973 in Belgium.
Ian M. Evans and Gwen Richards